Adhesive formulations capable of bonding to low energy surfaces such as polyolefins are now commonplace. The ability to adhesively bond to a surface with a limited number of available bonding sites and characterized by a surface energy value of less than approximately 48 miliJoules per meter squared (mJ/m2) has been addressed in the past through surface activation of the low energy surface through various treatments such as exposure to flame, plasma, ion bombardment, or other processes to create reactive moieties to which an adhesive could bond. While such low energy surface modification treatments proved effective, they have met with limited acceptance owing to the cost, limited duration of surface activation, and the impracticality of surface treatment in field usage or to bond large area substrates.
Resort to primer compositions intermediate between a low energy surface and an adhesive were found to address in part the limitations of high energy surface treatments, yet such primers add to the cost and complexity of bonding thereby limiting instances of practical usage. Additionally, the strength of low energy surfaces adhesively bonded through resort to primers has compromised strength owing to interfacial delamination.
In response to these limitations, adhesive formulations have been developed that rely on organoboranes as free radical polymerization initiators to induce cure of an adhesive formulation and simultaneously promote adhesive bonding to a contacting substrate. Exemplary of such compositions are those detailed in U.S. Pat. Nos. 5,106,928; 6,706,831; and 5,935,711. Organoborane amine complexes overcame many of the stability issues associated with organoboranes and represent the state-of-the art in adhesive bonding to low energy surfaces. Unfortunately, while organoborane amine complex formulations overcome many of the aforementioned problems of energy surface treatments, primers, and unstable organoboranes, persistent limitations of these formulations have led to limited market acceptance. Existing adhesive formulations are two-part formulations that require a mix weight ratio of adhesive Part A to activator Part B of 4:1 ratio, which represents an inconvenient mix ratio for many applications. Additionally, conventional organoborane amine complex formulations have a slower than desired cure rate with adhesive strength developing slowly as evidenced by a single lap shear strength of 345 kilopascals (kPa) taking approximately two hours to develop, as measured by ASTM D 1002 at standard temperature and pressure (STP). Additionally these conventional formulations suffer from poor storage thermal stability at elevated temperatures of above 40° C. that are often experienced by adhesive formulations prior to usage.
Thus, there exists a need for a low surface energy bonding adhesive formulation able to develop initial strength more quickly than conventional formulations, and to do so without resort to prior low surface energy substrate treatment. There also exists a need for such a formulation that has superior thermal stability to promote formulation storage prior to usage.